BEFORE, you learned Most earthquakes occur along tectonic plate boundaries Different directions of stress cause normal, reverse, and strike-slip faults.

BEFORE, you learned Most earthquakes occur along tectonic plate boundaries Different directions of stress cause normal, reverse, and strike-slip faults NOW, you will learn How energy from an earthquake travels through Earth How an earthquake's location is determined Magnet Word Diagram= Include: definition, example(s), picture, pronounce, origin, sentence. -seismic wave -focus -epicenter -seismograph Energy from earthquakes travels through Earth. When you throw a rock into a pond, waves ripple outward from the spot where the rock hits the water. The energy released by an earthquake travels in a similar way through Earth. Unlike the pond ripples, though, earthquake energy travels outward in all directionsup, down, and to the sides. The energy travels as seismic waves, (SYZ-mihk) which are vibrations caused by earthquakes. Seismic waves from even small earthquakes can be recorded by sensitive instruments around the world. The prefix epi- comes from a Greek word meaning on top of. An earthquake's epicenter is directly over its focus. All earthquakes start beneath Earth's surface. The focus of an earthquake is the point underground where rocks first begin to move. Seismic waves travel outward from the earthquake's focus. The epicenter (EHP-ih- SEHN-tuhr) is the point on Earth's surface directly above the focus. Scientists often name an earthquake after the city that is closest to its epicenter. In general, if two earthquakes of equal strength have the same epicenter, the one with the shallower focus causes more damage. Seismic waves from a deep-focus earthquake lose more of their energy as they travel farther up to Earth's surface. The depths of earthquakes along tectonic plate boundaries are related to the directions in which the plates move. For example, an earthquake along a mid-ocean spreading center has a shallow focus. There, the plates are pulling apart, and the new crust that forms is thin. Subduction zones have a wide range of earthquake depths, from shallow to very deep. Earthquakes can occur anywhere along the sinking plates. Waves and Energy Waves are part of your everyday life. For example, music reaches your ears as sound waves. All waves, including seismic waves, carry energy from place to place. As a wave moves through a material, particles of the material move out of position temporarily, causing the particles next to them to move. After each particle moves, it returns to its original position. In this way, energy moves through the material, but matter does not. On October 17, 1989, an earthquake stopped baseball's World Series at Candlestick Park in San Francisco. As the seismic waves arrived, fans heard a low rumble; then for about 15 seconds the stadium shook from side to side and up and down. About 20 minutes after the earthquake was felt at the stadium, the seismic waves had traveled to the other side of Earth. There, the waves did not shake the ground hard enough for people to notice. The waves could be detected only by scientific instruments. Waves and Energy (cont.) Earthquakes produce three types of seismic waves: primary waves, secondary waves, and surface waves. Each type moves through materials differently. In addition, the waves can reflect, or bounce, off boundaries between different layers. The waves can also bend as they pass from one layer into another. Scientists learn about Earth's layers by studying the paths and speeds of seismic waves traveling through Earth. Primary Waves The fastest seismic waves are called primary waves, or P waves. These waves are the first to reach any particular location after an earthquake occurs. Primary waves travel through Earth's crust at an average speed of about 5 kilometers per second (3 mi/s). Primary waves can travel through solids, liquids, and gases. As they pass through a material, the particles of the material are slightly pushed together and pulled apart. Buildings also experience this push and pull as primary waves pass through the ground they are built on. *One meaning of primary is first. Primary waves arrive before secondary waves Secondary Waves Secondary waves are the second seismic waves to arrive at any particular location after an earthquake, though they start at the same time as primary waves. Secondary waves travel through Earth's interior at about half the speed of primary waves. Secondary waves are also called S waves. As they pass through a material, the material's particles are shaken up and down or from side to side. Secondary waves rock small buildings back and forth as they pass. Secondary waves can travel through rock, but unlike primary waves they cannot travel through liquids or gases. Look at the illustrations. As a primary wave passes through a material, the volume and density of the material change slightly. But as a secondary wave passes, the material changes slightly in shape. Liquids and gases do not have definite shapes. These materials flowthat is, particles in them do not return to their original positions after being moved. When scientists learned that secondary waves cannot pass through Earth's outer core, they realized that the outer core is not solid. Why can't secondary waves travel through liquids or gases? Surface Waves Surface waves are seismic waves that move along Earth's surface, not through its interior. They make the ground roll up and down or shake from side to side. Surface waves cause the largest ground movements and the most damage. Surface waves travel more slowly than the other types of seismic waves. Seismic waves can be measured. Without listening to the news, scientists at seismic stations all over the world know when an earthquake occurs. Seismic stations are places where ground movements are measured. A seismograph (SYZ-muh-GRAF) is an instrument that constantly records ground movements. The recording of an earthquake looks like a group of wiggles in a line. The height of the wiggles indicates the amount of ground movement produced by seismic waves at the seismograph's location. Why is more than one kind of seismograph needed to record all the movements of the ground during an earthquake? Using Seismographs Separate seismographs are needed to record side-to-side movements and up-and-down movements. A seismograph that measures side- to-side movements has a heavy weight hanging from a wire. The weight remains almost still as the ground moves back and forth beneath it. A pen attached to the weight records the movements. A seismograph that records up-and- down movements has a heavy weight hanging from a spring. As the ground moves, the weight stays almost still as the spring absorbs the movement by getting longer or shorter. A pen attached to the weight records the changes in distance between the ground and the weight. Scientists use seismographs to measure thousands of earthquakes, large and small, every year. Some seismographs can detect ground movements as small as one hundred-millionth of a centimeter. The recording produced by a seismograph is called a seismogram. By studying seismograms, scientists can determine the locations and strengths of earthquakes. Locating an Earthquake To locate the epicenter of an earthquake, scientists must have seismograms from at least three seismic stations. The procedure for locating an epicenter has three steps: 1. Scientists find the difference between the arrival times of the primary and the secondary waves at each of the three stations. 2. The time difference is used to determine the distance of the epicenter from each station. The greater the difference in time, the farther away the epicenter is. 3. A circle is drawn around each station, with a radius corresponding to the epicenter's distance from that station. The point where the three circles meet is the epicenter. Scientists can also use seismograph data to locate the focus of an earthquake. They study seismograms to identify waves that have reflected off boundaries inside Earth. Some of these waves help the scientists to determine the earthquake's depth. A seismogram records the time when the first primary wave arrives. This wave travels by a direct path. The data also show when the first reflected primary wave arrives. After leaving the focus, this wave reflects from Earth's surface and then travels to the seismic station. The reflected wave takes a longer path, so it arrives slightly later. The difference in arrival times indicates the depth of the focus. Scientists can make the necessary calculations, but more commonly a computer is used to calculate the location of an earthquake's epicenter and focus. Scientists also use seismograms to determine earthquakes' magnitudes, or strengths. The more energy an earthquake releases, the greater the ground movement recorded. The greatest movement determines the earthquake's strength on a magnitude scale. Stronger earthquakes get higher numbers. You will read more about earthquake magnitude scales in the next section. KEY CONCEPTS 1.Why does the greatest shaking of the ground occur near an earthquake's epicenter? 2. What information do you need to completely describe where an earthquake started? 3. What types of information can a scientist get by studying seismograms? CRITICAL THINKING 4. Compare and Contrast How are primary and secondary waves similar? How are they different? 5. Apply What information could you get about an earthquake's location from only two seismic stations' data? Explain. CHALLENGE 6. Apply Why might an earthquake's primary waves, but not its secondary waves, reach a location on the other side of the world from the epicenter?